Air shield for combustor firetube stack

ABSTRACT

A combustor having an outer stack placed around a fire tube of the combustor to direct cooler air upwards to envelop an exhaust plume from the fire tube as an air shield or curtain barrier to prevent fugitive gases coming into contact with the exhaust plume and auto-igniting.

PRIORITY STATEMENT & CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from co-pending U.S. Patent Application No. 62/949,293, entitled “Air Shield for Combustor Firetube Stack” and filed on Dec. 17, 2019, in the name of Chris Aldrich; which is hereby incorporated by reference, in entirety, for all purposes.

TECHNICAL FIELD OF THE INVENTION

This invention relates, in general, to and, in particular, to the field of shielding combustor stack exhaust plumes from fugitive gases, in particular, air shield barriers for shielding combustor stack exhaust plumes from fugitive gases.

BACKGROUND OF THE INVENTION

Many tanks for storing liquids, particularly in the oil and gas industry, are associated with combustible vapors, sometimes referred to as “fugitive gases”. Examples of fugitive gases include methane, ethane, propane in addition to other hydrocarbon gases as well known to those skilled in the art. Such tanks include, but are not limited to, oil production tanks, produced water and condensate tanks and the like. Hydrocarbon vapors collect in the head space of the tank. The tanks might be vented to atmosphere, but more preferably, for environmental and possibly regulatory reasons, the vapors need to be eliminated. If the vapors are vented to atmosphere, and not eliminated through a combustion process, then this can also add to the greenhouse gas problem which is associated with climate change.

Such tanks are usually operated at low pressures, such as just over atmospheric, the pressure therein numbering in mere inches of water column. Optional pumps or vapor recovery units can be used to raise scavenged vapors to higher pressure, for delivery to be sent into sales gas lines for custody transfer or used on heating appliances as the burner fuel gas. Combustion of low vapor gases is typically a continuous process, in burners configured for low pressure fuel and air.

Natural draft burners are used in a variety of process apparatus, line heaters, reboilers, heat treaters, free water knockout drums, storage tanks and the like, commonly used in the oil and gas industry. Such equipment is often located on oil and gas sites, or other industrial sites having the possibility of fugitive hydrocarbons present, and at which ignition sources are carefully controlled. These burners can be further used in a combustor to burn off waste gases located at an oil or gas production or storage facility. One or more natural draft burners can be supported in a firetube of a combustor that can extend upwards wherein the air heated as a result of the combustion of waste gases can rise and exit into the atmosphere.

One problem that exists is that some fugitive emissions on an oil and gas well site can accumulate in the vicinity of a combustor and blow over and into an exhaust plume exiting from a combustor firetube and auto-ignite as a result of the temperature of the exhaust plume being higher than the auto-ignition temperature of the fugitive gas. The temperature of the exhaust plume can be in excess of 800 degrees Celsius, which is more than sufficient to auto-ignite most, if not all, fugitive gases.

Some well sites can have small footprints thus placing a combustor in close proximity to oil and gas production equipment or storage equipment and increasing the risk of ignition of fugitive gases. Without having a combustor on a well site to burn off the low pressure and/or low volume gases, then these gases would then be vented to the atmosphere and contribute to the accumulation of greenhouse gases in the atmosphere.

Another reason for having a combustor in close proximity to oil and gas production equipment or storage equipment is to minimize line loss in the pipes feeding waste gas to the combustor. This allows for the use of 3 inch or 4 inch pipe to feed the waste gas to the combustor. If the combustor is placed further away from the equipment, then the low pressure and/or low volume of the gas would require larger diameter pipe, such as 6 inch or more, to compensate for the line loss associated with longer pipe runs to feed the waste gas to the combustor. This raises other problems such as condensation forming inside the pipe, which could freeze in sub-zero degree Celsius temperatures that, in turn, would require heat tracing to be placed on the pipe to prevent the freezing.

It is, therefore, desirable to provide a combustor that can be placed in close proximity to oil and gas production and storage facilities and equipment for destroying waste gases that would be otherwise vented into the atmosphere and not auto-ignite fugitive gases that may be present at the site.

SUMMARY OF THE INVENTION

An air shield for shielding exhaust plumes from a combustor exhaust stack can be provided. In some embodiments, the air shield can comprise of an outer stack or jacket disposed around the exhaust stack or inner firetube of a combustor where the inner firetube extends slightly higher than the end of the outer stack, and wherein an annular chamber is formed between the inner firetube and the outer stack. In some embodiments, the annular chamber and the inner firetube are in communication with a combustor base box that can draw in air through one or more flame arrestors. In some embodiments, the combustor box can comprise a burner assembly that is configured to burn low pressure and/or low volume of fugitive gas. Some examples of suitable burner assemblies are disclosed in U.S. Pat. Nos. 9,709,266 and 10,247,411, which are incorporated by reference into this application in their entirety.

In some embodiments, when the burner assembly is operating, air is drawn into the combustor box through the flame arrestors to be used as combustion air for the burner assembly resulting in heated air rising in the inner firetube as a heated exhaust plume, which can reach temperatures higher than the auto-ignition temperature of any fugitive gas. Air drawn into the combustor box can also enter the annular chamber and becomes heated by the inner firetube and can then rise up the annular chamber until it exits as shielding air at the top of the outer jacket just below the top of the inner firetube. The exhaust plume exiting the inner firetube can further draw the shielding air exiting the annular chamber, which is cooler than the exhaust plume, to surround the exhaust plume as an air shield or curtain. As a result, any fugitive gases in the vicinity of the exiting exhaust plume can be prevented from being exposed to the high temperature of the exhaust plume that can cause the fugitive gas to auto-ignite. In some embodiments, the firetube can comprise one or more air inlets disposed along its length and/or around its circumference, wherein cooler shielding air can be drawn from the annular chamber into the firetube and mix with the heated air rising in the firetube, which can further cool the exhaust plume as it exits the firetube.

Broadly stated, in some embodiments, a method can be provided for shielding an exhaust plume exiting an outlet of a firetube of a combustor to prevent fugitive gases in the surrounding atmosphere from auto-igniting, the combustor comprising a base combustor box, the firetube comprising an inlet disposed on top of the combustor box wherein the inlet is in communication with an interior of the combustor box, a burner assembly disposed in the inlet, one or more flame arrestors disposed on the combustor box configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the method comprising: drawing in shielding air into the combustor box; conveying the shielding air from the combustor box along an exterior of the firetube up to the outlet of the firetube; and enveloping the exhaust plume with the shielding air to form an air shield therearound, whereby the air shield forms a barrier between the exhaust plume and the fugitive gases to prevent the fugitive gases from auto-igniting.

Broadly stated, in some embodiments, the method can further comprise drawing in a portion of the shielding air into the firetube at one or more locations disposed along the firetube.

Broadly stated, in some embodiments, wherein the conveying of the shielding air can comprise natural draft or convection.

Broadly stated, in some embodiments, an apparatus can be provided for shielding an exhaust plume exiting an outlet of a firetube of a combustor to prevent fugitive gases in the surrounding atmosphere from auto-igniting, the combustor comprising a base combustor box, a burner assembly disposed in the combustor box, one or more flame arrestors disposed on the combustor configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the apparatus comprising: an outer stack or jacket disposed around the firetube thereby forming an annular chamber therearound, the annular chamber in communication with the atmospheric air disposed in the combustor box, wherein the atmospheric air is drawn into the annular chamber to form shielding air.

Broadly stated, in some embodiments, an improved combustor can be provided, the combustor comprising a base combustor box, a burner assembly disposed in the combustor box, one or more flame arrestors disposed on the combustor configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the improvement comprising: an outer stack or jacket disposed around the firetube thereby forming an annular chamber therearound, the annular chamber in communication with the atmospheric air disposed in the combustor box, wherein the atmospheric air is drawn into the annular chamber to form shielding air.

Broadly stated, in some embodiments, a top end of the outer stack can be disposed below the outlet of the firetube.

Broadly stated, in some embodiments, the firetube can comprise one or more breather openings configured to allow the shielding air to be drawn into the firetube.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:

FIG. 1A is a side elevation view depicting one embodiment of a combustor with an exhaust plume air shield.

FIG. 1B is a temperature legend depicting the temperatures associated with the exhaust plume of the combustor of FIG. 1A

FIG. 2 is a top plan view depicting the stack of the combustor of FIG. 1A showing temperature measurement locations

FIG. 3 is a chart depicting a temperature profile of the exhaust plume of the combustor of FIG. 1B.

FIG. 4A is a top plan view depicting the combustor of FIG. 1A.

FIG. 4B is a side elevation cross section view depicting the combustor of FIG. 4A along section lines A-A.

FIG. 5A is a front elevation view depicting the combustor base box of the combustor of FIG. 1A.

FIG. 5B is a side elevation view depicting the combustor base box of FIG. 5A.

FIG. 5C is a bottom plan view depicting the combustor base box of FIG. 5A.

FIG. 5D is a top plan view depicting the combustor base box of FIG. 5A.

FIG. 5E is a rear elevation view depicting the combustor base box of FIG. 5A.

FIG. 5F is a cross section view depicting the tubing frame of the combustor base box of FIG. 5A.

FIG. 6 is a side elevation view depicting a base plate gusset of the combustor base box of FIG. 5A.

FIG. 7 is a side elevation view depicting an internal gusset of the combustor base box of FIG. 5D.

FIG. 8 is an elevation view depicting the assembly layout of the combustor of FIG. 1A.

FIG. 9 is a top plan view depicting the outer stack of the combustor of FIG. 8.

FIG. 10 is a side elevation view depicting a lifting lug of the combustor of FIG. 8.

FIG. 11 is a side elevation view depicting a heat guard mount of the combustor of FIG. 8.

FIG. 12 is a side elevation view depicting an outer stack support gusset of the combustor of FIG. 8.

FIG. 13 is an elevation view depicting the firetube of the combustor of FIG. 1A.

FIG. 14 is an elevation view depicting a breather notch of the firetube of FIG. 13.

FIG. 15 is a top plan view depicting the outer stack and firetube of the combustor of FIG. 8.

FIG. 16 is a bottom plan view depicting the outer stack and firetube of the combustor of FIG. 15.

FIG. 17 comprises side and end views depicting a support bracket.

FIG. 18 is a side elevation cross section view depicting the mounting of the firetube to the outer stack using the support bracket of FIG. 17.

FIG. 19 is a side elevation view depicting the top support bracket of the firetube of FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment can also be included in other embodiments but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein.

Referring to the Figures, an embodiment of an improved combustor is shown. In some embodiments, combustor 10 can comprise of base box 20, firetube 12 disposed on top of base box 20 and outer stack disposed circumferentially around firetube 12 and on top of base box 20 to form annular space 16 disposed between firetube 12 and outer stack 14.

Referring to FIGS. 5A to 5E, base box 20 can comprise a cube-like structure made of plate steel and can further comprise base box 20 disposed on base plate 36, and further supported by gussets 40 welded therebetween on each corner thereof. Base box 20 can further comprise internal gussets 42 welded in each upper internal corner thereof, as illustrated in FIG. 5D. In some embodiments, base box 20 can further comprise flame arrestors 22 disposed on opposing sides thereof to allow air to enter therethrough into base box 20. In some embodiments, base box 20 can comprise access door 24 disposed on a side of base box 20 to permit ingress thereto by service personnel. Access door 24 can be attached to door frame 34, which can be comprised of square steel tubing, with hinges and/or fasteners as well known to those skilled in the art. Base box 20 can further comprise opening disposed through top surface 21 thereof to provide communication to firetube 12 and outer stack 14 when mounted on top surface 21.

Referring to FIGS. 8 to 12, in some embodiments, outer stack 14 can be mounted on top surface 21, centered on opening 38, by welding or other fastening means as well known to those skilled in the art. Outer stack 14 can be further supported on top surface 21 by gussets 46 welded therebetween in a spaced-apart configuration around the circumference of outer stack 14. In some embodiments, outer stack 14 can have heat guard 18 mounted around the outside of outer stack 14, as shown in FIG. 8, with a plurality of heat guard mounts 48. In some embodiments, outer stack 14 can comprise a plurality of lifting lugs 44 mounted around the circumference of outer stack 14 in a spaced-apart configuration to allow combustor 10 to be picked up and moved by a crane or other suitable means.

Referring to FIGS. 13 to 19, in some embodiments, firetube 12 can be disposed within outer stack 14, wherein firetube 12 can be attached to, and supported by, a plurality of support brackets 50 disposed at a lower end of firetube 12, where threaded fasteners (not shown) can be used to secure firetube 12 to outer stack 14. In some embodiments, firetube 12 can be further supported and substantially centered within outer stack 14 by a plurality of top support brackets 52 disposed around the circumference of an upper end of firetube 12 in a spaced-apart configuration. By mounting firetube 12 within outer stack 14 in this configuration, annular chamber 16 is formed, which can be in communication with the interior of base box 20. In some embodiments, firetube 12 can extend upwards in height above the upper end of outer stack 14. In a representative embodiment, wherein outer stack 14 can be 36 inches in diameter, nominally, and where firetube 12 can be 30 inches in diameter, nominally, firetube 12 can extend above outer stack 14 by 4 inches, approximately.

In some embodiments, firetube 12 can comprise a plurality of notch openings 28 disposed around the circumference thereof and at different elevations thereof to permit the influx of air from annular chamber 16 when combustor 10 is in operation. In some embodiments, notch openings 28 can be approximately rectangular in shape and be formed by cuts through firetube 12 along the sides and bottom edge of the rectangular shape and with the cut portion bent outwards along the top edge thereof.

Referring to FIGS. 4A and 4B, in some embodiments, combustor 10 can comprise burner assembly 30 disposed within firetube 12 at a lower end thereof, where burner assembly 30 can be supplied by inlet waste gas to be burned supplied gas inlet 26. When combustor 10 is in operation, burner assembly 30 can be ignited to burn waste gas entering through gas inlet 26 using air within base box 20 as combustion air, with the resulting heated air rising up firetube 12. Air can be further drawn into base box 20 through flame arresters 22 to provide the combustion air. Air can also enter in annular chamber 16 to form shielding air 17 (as shown in FIG. 1A). The shielding air can rise in annular chamber 16 as a result of natural draft or convection therethrough. A portion of the shielding air can be drawn into firetube 12 through notch openings 28, as described above. As exhaust plume 13 (as shown in FIG. 1A) exits the upper end of firetube 12, shielding air 17 can be drawn up from annular chamber 16 through natural draft or convection and envelop exhaust plume 13 with a shield or curtain of cooler air, which can act as a barrier between exhaust plume 13 and any fugitive gases that may be present in the atmosphere surrounding combustor 10. In some embodiments, combustor 10 can comprise thermocouple 32 operatively coupled to the exterior of outer stack 14, which can be configured to measure the temperature thereof. Thermocouple 32 can be operatively coupled to a control system (not shown) wherein the control system can be configured to shut down the operation of combustor 10 should the temperature of outer stack 14 exceed a safe operating temperature as well known to those skilled in the art.

Referring to FIGS. 1A, 1B, 2 and 3, these figures illustrate the results of a test in measuring the temperature of exhaust plume at various locations radially from the center of firetube 12 and vertically from the upper end of firetube 12. In the test, the temperature of exhaust plume 13 at the center of firetube 12 was measured to be 825 degrees Celsius right at the mouth of firetube 12. At 36 inches higher in elevation from firetube 12, the temperature was measured at 75 degrees Celsius. The temperature exhaust plume 13 at locations 16½ inches radially from the center of firetube 12 range from 39 degrees to 96 degrees Celsius. This illustrates that shielding air 17 can provide an air shield or curtain that can surround or envelop exhaust plume 13 where the temperature at various locations within shielding air 17 can be kept below the auto-ignition temperature of a fugitive gas. Thus, shielding air 17 can act as a barrier between exhaust plume 13 and any fugitive gas in the atmosphere surrounding combustor 10.

Although a few embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications can be made to these embodiments without changing or departing from their scope, intent or functionality. The terms and expressions used in the preceding specification have been used herein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the invention is defined and limited only by the claims that follow. 

What is claimed is:
 1. A method for shielding an exhaust plume exiting an outlet of a firetube of a combustor to prevent fugitive gases in the surrounding atmosphere from auto-igniting, the combustor comprising a base combustor box, the firetube comprising an inlet disposed on top of the combustor box wherein the inlet is in communication with an interior of the combustor box, a burner assembly disposed in the inlet, one or more flame arrestors disposed on the combustor box configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the method comprising: drawing in shielding air into the combustor box; conveying the shielding air from the combustor box along an exterior of the firetube up to the outlet of the firetube; and enveloping the exhaust plume with the shielding air to form an air shield therearound, whereby the air shield forms a barrier between the exhaust plume and the fugitive gases to prevent the fugitive gases from auto-igniting.
 2. The method as set forth in claim 1, further comprising drawing in a portion of the shielding air into the firetube at one or more locations disposed along the firetube.
 3. The method as set forth claim 1, wherein the conveying of the shielding air comprises natural draft or convection.
 4. An apparatus for shielding an exhaust plume exiting an outlet of a firetube of a combustor to prevent fugitive gases in the surrounding atmosphere from auto-igniting, the combustor comprising a base combustor box, a burner assembly disposed in the combustor box, one or more flame arrestors disposed on the combustor configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the apparatus comprising: an outer stack or jacket disposed around the firetube thereby forming an annular chamber therearound, the annular chamber in communication with the atmospheric air disposed in the combustor box, wherein the atmospheric air is drawn into the annular chamber to form shielding air.
 5. The apparatus as set forth in claim 4, wherein a top end of the outer stack is disposed below the outlet of the firetube.
 6. The apparatus as set forth in claim 4, wherein the firetube comprises one or more breather openings configured to allow the shielding air to be drawn into the firetube.
 7. An improved combustor, the combustor comprising a base combustor box, a burner assembly disposed in the combustor box, one or more flame arrestors disposed on the combustor configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the improvement comprising: an outer stack or jacket disposed around the firetube thereby forming an annular chamber therearound, the annular chamber in communication with the atmospheric air disposed in the combustor box, wherein the atmospheric air is drawn into the annular chamber to form shielding air.
 8. The improved combustor as set forth in claim 7, wherein a top end of the outer stack is disposed below the outlet of the firetube.
 9. The improved combustor as set forth in claim 8, wherein the firetube comprises one or more breather openings configured to allow the shielding air to be drawn into the firetube. 